1. Field of the Invention
The invention relates to catalytic reduction of oxides of nitrogen, NO.sub.x, with ammonia.
2. Description of Related Art
The presence of NO.sub.x, or oxides of nitrogen, in flue gas streams, is a pervasive problem. Several powerful ways have been developed to deal with the problem. A non-catalytic approach, using carbonaceous substances to reduce NO.sub.x will be reviewed first, followed by a review of several ammonia-based processes.
In Green et al, U.S. Pat. No. 4,828,680, which is incorporated herein by reference, the level of NO.sub.x emissions from a fluidized catalytic cracking (FCC) unit was reduced by incorporating carbonaceous particles such as sponge coke or coal into the circulating inventory of cracking catalyst. The carbonaceous particle performed several functions, selectively absorbing metal contaminants in the feed and also reducing NO.sub.x emissions in certain instances.
This approach is most suited to FCC units, where large volumes of coal or coke-containing particles can be easily handled. Some modification of the FCC unit may be necessary, and the reduction in NO.sub.x emissions may not be as great as desired.
It is also known to react NO.sub.x in flue gas with NH3. NH3 is a very selective reducing agent, which does not react rapidly with the excess oxygen which may be present in the flue gas.
Two types of NH.sub.3 process have evolved, thermal and catalytic.
Thermal processes, such as the Exxon Thermal DeNO.sub.x process, generally operate as homogeneous gas-phase processes at very high temperatures, typically around 1550.degree.-1900.degree. F. More details of such a process are disclosed by Lyon, R. K., Int. J. Chem. Kinet., 3, 315, 1976, which is incorporated herein by reference.
The catalytic systems which have been developed usually operate at much lower temperatures, typically at 300.degree.-850.degree. F. These temperatures are typical of flue gas streams. Unfortunately, the catalysts used in these processes are readily fouled, or the process lines plugged, by catalyst fines which are an integral part of FCC regenerator flue gas.
Perhaps one reason why NH3 has not been added commercially to FCC regenerators is the possibility that the added ammonia, in the conditions experienced in the typical FCC regenerator, could actually increase the NO.sub.x emissions. NH.sub.3, in the presence of O.sub.2, at temperatures above 900.degree. F., can be oxidized to form NO.sub.x, when catalysts such as V.sub.2 O.sub.5 are present. This was reported by Bosch, H and Janssen, F, Catalysis Today, 4(2), 1987. Direct addition of such catalyst, V.sub.2 O.sub.5, to FCC regenerators is undesirable because such materials are poisons to the FCC process, and because the FCC regenerator environment, in the presence of such highly oxidized vanadium catalysts, could convert NH3 to NO.sub.x
I wanted a way to take advantage of the selective DeNO.sub.x capability of NH.sub.3, but without operating at the high temperatures required by the thermal systems, and using a catalyst system which would be compatible with the contaminants found in FCC regenerator and other similar flue gas streams. Ideally, I wanted a process which could be added on to, or incorporated into, an existing NO.sub.x generator, such as an FCC regenerator, without adding greatly to the capital cost of the unit. I especially wanted to have a way to safely add NH.sub.3 to an FCC regenerator flue gas stream, in a way which would reduce NO.sub.x emissions, without intermittently adding NH3 to the atmosphere as unit operation changed.
I discovered that the reduction of NO.sub.x emissions by contact with NH.sub.3 could be greatly improved in an FCC unit by adding a NO.sub.x conversion catalyst. With the use of an NO.sub.x conversion catalyst, the process became one which could be used for stack gas cleanup in general, and which was especially useful in FCC units.